Method of making split piston rings



Nov. 9, 1948. L. c. MARSHALL I METHOD OF MAKING SPLIT PISTON RINGS,

3 Shets-Sheet 1 Filed Dec. 14, 1945 Imam, @4660! 6: Marsha L. c. MARQHALL METHOD OF MAKING SPLIT PISTON RINGS Nov. 9, 1948.

3 Sheets-Sheet 2 Filed Dec.

mania? lazuli; a dram M J 4% 3 Shets-Sheet 3 1948- L. c. MARSHALL METHOD OF MAKING, SPLIT PISTON RINGS Filed Dec. 14, 1945 Patented Nov. 9, 1948 UNITED STATES PATENT OFFICE METHOD OF MAKING SPLIT PISTON RINGS Lewis C. Marshall, East Walpole, Mass. Application December 14, 1945, Serial No. 635,004

3 Claims.

The invention pertains to split piston rings, such as are used in the cylinders of internal combustion engines, for example, and more especially to a method of making such rings so that they will exert substantially uniform radial pressure upon the cylinder wall when in use, and to a novel ring resultant from the practice of such method. One of the principal difiiculties encountered in the manufacture of split piston rings is that of so designing a split ring that when in use it will exert a uniform radial pressure against the cylinder wall. Rings of non-uniform radial section have been proposed as a means of overcoming this difficulty. It has also been proposed to employ an internal expander ring in association with the contact ring in order to supply the requisite expansive force and to secure uniform radial pressure. However, the manufacture of rings of non-uniform but accurate transverse section is difiicult and expensive, while the employment of expander rings in association with contact rings involves diificulty in installation, in addition to the expense involved in making such composite rings.

In my prior Patent No. 1,783,047, dated November 25, 1930, I describe a method of making piston rings from steel wire by bending a length of wire or rod of uniform cross section and consistng of steel of from 0.5% to 0.8% carbon so as to form a single angular turn, severing said turn from the length of material to form an embryo ring having a gap at one point, and heating it sufficiently (while confining it both circumferentially and axially) to impart a permanent set, thereby to provide a ring of predetermined contour. The aforesaid patent suggests that by heating the ring while it is mounted upon and conformed to a mandrel of slightly elliptical transverse section, the finished ring, when placed in the cylinder, will exert approximately uniform radial pressure against the cylinder wall. While such a ring of approximately elliptical contour does exert a more uniform pressure against the cylinder wall than rings of true circular contour, an elliptical contour does not provide exact uniformity of radial pressure. I have now discovered a procedure whereby it is possible to impart to the uncompressed or open ring such a contour that when it is installed in a cylinder it actually exerts a uniform pressure against the cylinder wall throughout its entire periphery. The life of the ring and also that of the cylinder is thus prolonged; the passage of oil is reduced to a minimum; a better compression is attained, and it is unnecessary to resort to the use of auxiliary expander rings or the like in order to secure the desired radial pressure. The best known mode of obtaining this result is hereinafter more fully described by reference to the accompanying drawings wherein Fig. 1 is a diagrammatic plan View of a half ring showing the ring in its closed position in dotted lines and in its open or unstressed condition in full lines and indicating the deviation of a given point in the open ring from the corresponding position of said point in the closed or stressed ring;

Fig. 2 is a plan view, to smaller scale, of a split ring of the type to which the present invention relates, the ring being shown open or unstressed;

Fig. 3 is an edge view of the ring of Fig. 2;

Fig. 4 is another diagram showing a half ring in the closed or stressed condition and indicating the application of some of the mathematical terms employed in the discussion; and

Figs. 5 and 6 are tables of values representing the practical application of the present invention to the proper shaping of rings of specific dimensions.

When the terms horizontal and vertical" are herein employed, they are used in a mathematical sense in defining the shape of the ring by reference to rectangular coordinates, with the axis of coordinates at the center of the ring and with the horizontal axis of coordinates bisecting the gap in the ring.

In accordance with the theory of elasticity, a beam of uniform transverse section fixed at one end and whose axis forms a circular arc, will deflect, when uniformly loaded with a radially acting load, according to the following Equations 1 and 2.

In these equationsrm=the mean radius of the beam.

ra=the outside radius.

M=the moment of the load.

J :the moment of inertia of the ring section.

E=the modulus of elasticity of the ring material.

p=radial pressure per square inch acting on the mean radius of the closed ring, and bTmdqJ is the infinitesimal area at A corresponding to the infinitely small angle d Aa:=the horizontal displacement from a true circle of any point P in the loaded beam at an angle Illa from the fixed end, and A the vertical displacement from a true circle of the same point in the loaded beam at an angle the. from the fixed end.

For a piston ring made from material, for instance steel wire, of uniform rectangular transverse section, the moment of inertia is constant where b=the axial thickness of the ring and h=the radial thickness oi the ring.

With the above definitions of the terms employed, and referring to Figs. 1 and 4,

tion B due to the load upon the are BID as given by the equation Q=P ZL sin =p 1+ o a) This latter equation determines the moment at any section B (corresponding to the angle be) due to the uniform radial load acting on the mean circumference.

Substituting the value Integrating Equations 5 and 6, and substituting cos t where pa is the uniform radial pressure at the outer circumference (Ta being the outside radius)- o 3 (10 A1 2% -A,

Equations 9 and 10 define the deviation from a true circular arc of any point P (Fig. 1) at an angle be measured from the fixed end of the beam and resultant from uniform loading.

Conversely, if a piston ring be so shaped, that when open or unstressed, every point in its circumference is displaced from a true circle so that the coordinates A0: and Ag of each such point are defined by Equations 9 and 10 it follows that when the ring is closed and confined within the cylinder, it will exert a, uniform outward radial pressure pa against the cylinder wall.

Thus, if the modulus of elasticity E of the material or" the ring be known, and ii"- the ring be of substantially uniform rectangular radial section, its open or unstressed shape may readily be determined by substituting the proper constants in Equations 9 and 10.

Specific examples of the shapes to be arrived at by the use of the above equations are shown in Figs. 5 and 6. Fig. 5, in tabular form, shows the departure from a true circular shape, of the unstressed or open ring, giving the radial distances measured outwardly from a true circle, for each degree measured from the mid-radius of the gap, the outside radius To. of the closed ring being 1.8125 inches; it equaling 0.103 inch; h equaling 0.12375 inch; F equaling 9.75 lbs; the radial pressure p equaling 14.5 lbs. per square inch; E equaling 26,290,000; and the diameter of the base circle from which the radial distances are measured equaling 3.547 inches.

' ikewise in te 'ular shape of the g, giving the radial distances a true or base circle for cm. the mid-radius of the Ta of the closed ring being 56 inches; it @qflm ng 0.199 inch; b equaling 805 in h; eq' 11mg 10 lbs; the radial pressure g 1. lbs. per square inch; E equaling and the diameter of the base circle ii'om which the radial distances are measured equaling 3.156 inch.

The values in the above tables which fall within the width of the gap be disregarded, but when preparing a heat treating mandrel for use in setting the ring to the required contour, it is desirable to shape the entire periphery of the man-- vdrel in accordance with the tabulated dimensions.

Having thus accurately determined the proper our of the unstressed or open ring, the con-- tour i ay be imp ted to the open ring any usual or desired ma 1 r. F r example, cast iron rings may be machined to this contour; steel rings may be formed as described for example in the patent to Marshall No. 1,783,047, and heat treated and set to shape, while open but confined about a noncircular mandrel contoured in accordance with Equations 9 and 10; or if the ring he moulded from plastic or other rnouldable material, then the die or mould cavity may be contoured in accordance with Equations 9 or 10.

W ile it is not essential. to the proper design or the 1' .ig as above described, it is of interest to note that ring designed in accordance with Equations and 10 may closed by the application. of a gle force F applied at a point 9% from the gap ere i equals 3bl'af3, this r 'tiou being deduced by a method of analysis ally similar to that employed in arriving at Equations 9 or 10.

I claim:

1. Method of making a split piston ring operative to exert substantially uniform radial pressure against the cylinder wall when installed in the cylinder and which. when. so installed, has an out side radius To. and a mean radius Tm, and wherein an embryo ring is first formed by bending a length of steel wire to form an annular turn, cutting off angle 180 -'I//a from the mid-radius of the location of the ring-gap on the mandrel being in accordance with the equations mm?" AFWXAI and where E is the modulus of elasticity of the ring material, his the radial thickness of the ring, and where pa is the desired uniform pressure in pounds per square inch at the outside circumference of the ring, and

Ag X A 2 A,=la sin wa- -wos do and 2. Method of making a split piston ring operative to exert substantial uniform radial pressure against the cylinder wall when installed therein and which will have an outside radius Ta and a mean radius Tm when installed in the cylinder, said method comprising as steps forming an mbryo ring from material having :a modulus of elasticity E and of uniform cross section, the radial thickness of the material of the ring being h, and permanently deforming the ring from a true circle to the extent that the horizontal and vertical deviations Ar and Ag, respectively, from a true circle, of any point in the uncompressed ring will be in accordance with the equations and 12p T rf, Eh

where 17a is the desired uniform pressure in pounds per square inch at the outside circumference of the ring, and

and

3. Method of making a split piston ring 09- erative to exert substantially uniform radial pressure against the cylinder wall when installed therein and which is of substantially uniform radial section and of a material having a modulus of elasticity E and an outside radius Ta and a mean radius Tm when installed in the cylinder, said method comprising as a step the formation of an open unstressed but elastic ring, of radial thickmess it which deviates from a true circle, the horizontal and vertical deviations Arc and Ag, respectively, from a true circle, of any point in the unstressed open ring being in accordance with the equations where pa is the desired uniform pressure in pounds per square inch at the outside circumference of the ring, and

2 Azc=l-a sin il/a cos pa and LEWIS C. MARSHALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,314,534 Mummert Se t, 2, 1919 1,321,539 Mummert Nov. 11, 1919 1,661,366 Graves Mar. 6, 1928 1,732,830 Bennet Oct. 22, 1929 1,933,568 Six Nov. 7, 1933 2,061,780 Short Nov. 24, 1936 2,093,263 Wuerfel Sept. 14, 1937 

